Simulation of the real world is implemented in a wide variety of applications ranging from vehicle training simulators to video games. The simplest simulators are those used, for example, in video games. Generally, such simulators utilize a video display presented on a cathode ray tube monitor or a liquid crystal display. Such displays are meant to simulate the appearance of an outside world view seen through a window. The analog of the real world window in a display is, for example, the face of the liquid crystal display. The display has dimensionality, motion characteristics and perspective characteristics similar to those of a video image produced by a video camera. Accordingly, images presented on these sorts of systems lack realistic image change responses to head movement, eye movement and the like.
For example, if one viewing the real world through a window moves one's head to the right or the left, an image which is at a great distance from the observer will appear to move to the right or left within the window. In other words, as the viewer moves his head, the scene through the window will change and the object will appear to be stationary even though the observer is moving. This is in contrast to a directly viewed cathode-ray tube display, such as that produced on a personal computer or television set, where the image is independent of user head movement.
A similar effect obtains in the case of user movement toward and away from the display. Such user movement results in radical changes in apparent image size. This is in contrast to the real world where objects at a relatively large distance (for example 100 meters) appear unchanged in size despite relatively small (for example 5-10 centimeters) head movements toward and away from the display.
The problem stems from the fact that the simulated scene on a display is located a relatively finite distance from the user, perhaps 40 cm. Accordingly, head movement of a few centimeters causes radical disparity between expected image position and actual image position. Nonetheless, such systems do enjoy a large measure of popularity in the civilian world, because the human brain is well trained by television to accept such displays. Nevertheless, such displays, while accepted by the human brain, fail to provide the numerous visual cues such as image movement, parallax, image size and the like, that inform and then guide reactions.
Accordingly, merely providing a picture-type display is not suitable for experience-based training, such as that required by aircraft pilots, watercraft pilots, seaman and the like, fighting vehicle drivers, astronauts, drivers, and so forth, as well as gunnery, navigation and other personnel associated with vehicles and fixed installations such as bunkers, communication facilities and defensive/offensive gun emplacements. In addition remote prescence systems requiring real world image response to movements of the viewer.
One approach to the problem is the use of focusing optics which receive light from an image source and collimate it, making it appear that the image source is at a, or, at least, close to a great distance away. Because light coming from optics which are at a great distance is substantially parallel, collimation of an image source before presentation to the eye of a viewer is an effective strategy to present the viewer with effective real world simulation which will be useful to train reflexes, responses, and the like.
Such displays generally involve the placement of the image source near the focal point of a focusing optic. For example, if the image source is the face of a cathode ray tube (CRT), the CRT face plate may be positioned in the focal plane of the focusing optic. Accordingly, all light emanating from the face of the cathode ray tube and collected by the focusing optic will be presented as collimated, that is parallel bundles of light. While, in principle, refractive optics can perform this function, as a practical matter, the weight, thickness, and aberrations associated with refractive optics renders such an approach impractical. Accordingly, it is desirable to use reflective optics.
However, reflective optics present a multitude of challenges. For example, an image would generally be formed by having the source on the same side of the reflective optic as the observer. If the image is presented by folding the axis by the use of a partially reflective, partially transmissive planar optic, the assembly is clumsy and the presentation of adjacent assemblies complicated or impossible. While, in principle, one may replace the concave optic with a half-silvered concave member, holographic equivalent thereof, or the like, the direct view of the original image may destroy the usefullness of the effect.
Since the 1960's, simulator displays have been available which address the above problems with varying degrees of success. For example, the collimating simulation optic sold under the trademark Pancake Window provided a solution to this problem.
Such systems are described in United States Pat. No. Re 27,356, and comprise a sandwich comprising a vertical polarizer, half-silvered concave optic, quarter-wave plate, planar beam-splitter, quarter-wave plate, and horizontal polarizer, which sandwich is coupled to an image source.
Generally, in accordance with this technology, an image source may be caused to pass light through a vertical polarizer. The light from the vertical polarizer passes through a concave half-silvered focusing optic, which, in turn, passes image source light through a quarter-wave plate which gives the light clockwise (or right) circular polarization characteristics. The circularly polarized light is reflected off a beam-splitter (a half silvered planar mirror) which causes it to have reversed or counterclockwise circular polarization. The light which is now circularly polarized in a counterclockwise (or left) direction goes through the quarter-wave plate again, emerging as horizontally polarized light, which is, in turn, reflected by the concave reflective optic and passes through the quarter-wave plate again which restores it to a bundle of counterclockwise circular polarized light, which is transmitted by the beam-splitter with unaltered polarization and finally passed through a quarter wave plate which passes the light with a horizontal linear polarization characteristic. The output image is then passed through a horizontal polarizer. Light which passes directly from the image source through the vertical polarizer and through the various optical elements will be vertically polarized and will be blocked by the final polarizer which passes only the intended horizontally polarized light bundle.
Such systems have seen a wide variety of applications, including various vehicle simulators, such as aircraft simulators, space vehicle simulators, and so forth. Image sources used with such systems include cathode ray tube displays, models, painted displays, and so forth.
However, in theory, every transmission through and reflection off a half-silvered optical member and certain transmissions through a polarizer involve a nominal fifty percent loss of intensity. This loss occurs two times at the focusing optic, twice at the beam-splitter and once at the first of the two polarizers. Accordingly, significant losses occur, as appears more fully below.
In principle, some of the disadvantages of collimating optical members for simulators may be addressed by systems comprising a sandwich of elements comprising a vertical polarizer, a quarter wave plate, a concave half-silvered focusing optic, and a cholesteric polarizer. See, for example, U.S. Pat. No. 4,859,031. Generally, such systems work because the cholesteric polarizer has the characteristic of passing light with one circular polarization but transmitting light having the opposite circular polarization. However, such systems suffer from the inadequacy of the cholesteric polarizer which has a relatively narrow range of reflectivity and requires the use of, for example, three layers of optics, resulting in a relatively thick assembly forming the collimating optical simulator member, and presentation of images with multiple color components which are out of register with each other.